The invention relates generally to screen printing machines and indexes used for the semi-automatic printing of multicolor images.
In particular this invention relates to a pallet assembly in combination with a dual locator system that permits the efficient and accurate registration of a pallet prior to printing.
Modern day screen printing machines and associated indexers are adapted to index the objects to be printed upon between individual print stations for the purpose of receiving multicolor print images. Screen printing apparatuses of this general configuration are disclosed, for example, in U.S. Pat. Nos. Re 29,160, 4,031,825 and my co-pending application Ser. No. 118,429 filed on Nov. 6, 1987, the disclosures of which are hereby incorporated by reference.
In general, screen printing machines of the above-mentioned type are provided with a plurality of pallet support plates that surround and extend from a central drive mechanism. These plates are horizontally positioned on a track having a predetermined geometric configuration in a manner which permits co-planar movement of the support plates about the centrally located drive of the machine. The co-planar movement about the track is accomplished by a mechanical drive linked to the individual support plates by a flexible drive member such as a chain. The drive means, through a suitable indexing mechanism, produces the intermittent motion of the pallets between the various print stations. As would be understood by one skilled in the art, each print station has an associated print head attached to the frame of the printing apparatus so that the print head extends over the travelling plane of the support plates. As would be further understood by those skilled in the art, each print head contains the requisite apparatus for applying a single color print image to the article during the print cycle.
The articles to receive print images are placed onto individual pallets which are in turn fixably attached to the pallet support plates. In operation, the drive mechanism indexes the plates between the spaced print stations. When each plate arrives at its respective print station, its horizontal movement, and in some apparatuses its vertical movement is arrested momentarily. The support plate and associated pallet is then mechanically registered and locked in place and the print cycle begins. Printing is accomplished by pivoting the print head down, or otherwise causing a print screen to contact the surface of the article held by the pallet. A squeegee carriage and flood bar assembly traverse the surface of the screen thereby transferring the image on the screen onto the article. At the termination of the print stroke, the printing screen disengages the screen and squeegee from the work surface; horizontal movement of the pallets then resumes, indexing each pallet to the next print station for further printing.
In a multi-stage printing operation such as those described above, the quality of the print is critically contingent on the positional accuracy of the pallet at each station. If the pallet is permitted to drift from a pre-determined reference point, the quality of the print is adversely affected. In the past, attempts have been made to eliminate drift through the introduction of a registration step prior to the print cycle. This registration has been partly accomplished by providing a locking bar or similar device which is actuated to engage the outer edge of the pallet support plate at a single fixed location beneath the printing head. Similarly, registration of the inner edge of the pallet has been attempted by rigidly attaching the support plate to the flexible drive member.
Although the above-described methods of registration have reduced some lateral drift of the pallet, they have not eliminated it completely. Specifically, the registration means employing a single locator to fix the outer edge of the support plate is unable to restrict horizontal drift of the inner edge of the pallet. In this regard, the rigid attachment to the flexible drive member has not been completely satisfactory since the chain, due to is flexibility and tendency to stretch, imparts little, if any, positional control. Moreover, none of the prior art registration means satisfactorily address the problem of pallet drift in the direction transverse to the travel of the pallet, i.e., from front to rear of the printing indexer.